1. Field
The teachings in accordance with the exemplary embodiments of this present disclosure generally relate to a method for determining phase angle in phase shift transformer for medium voltage inverter.
2. Background
In general, a multilevel medium voltage inverter is an inverter having an input power whose rms (root mean square) value is over 600V for a line-to-line voltage, and has several stages in output phase voltage. The multilevel medium voltage inverter is generally used to drive an industrial load of large inertia ranging from several kW to several MW capacities of, for a non-limiting example, fans, pumps, compressors, tractions, hoists and conveyors.
The multilevel medium voltage inverter uses a phase shift transformer to reduce harmonics, where a phased shift angle of the phase shift transformer is determined by the number of unit power cells, and an increased number of unit power cells improve a THD (Total Harmonic Distortion) at an input terminal. However, if the number of unit power cells disadvantageously generate the THD at an input phase current.
FIG. 1 is a circuit diagram illustrating a configuration of a conventional multilevel medium voltage inverter, which is a schematic view illustrating a serially cascaded H-bridge multilevel inverter, and FIG. 2 is a schematic view illustrating a detailed configuration of unit power cells of FIG. 1.
A phase shift transformer (110) in a general multilevel medium inverter (100) changes phase and size of voltage in a high input power in response to requirement of a unit power cell (120). An output voltage of the phase shift transformer (110) is an input power of each unit power cell (120), and converted to a DC through a 3-phase diode rectifier (121).
FIGS. 3a and 3b illustrate a structure and a phase diagram of a phase shift transformer (110) where a phase shift angle at a secondary side is positive to a phase shift angle at a primary side (Y/Z-1), when the primary side of the phase shift transformer (110) is formed in a Y winding of N1 turn, a secondary side is formed in a delta (Δ) winding of N2 winding, and a tertiary side is formed in a winding of N3.
Furthermore, FIGS. 4a and 4b illustrate a structure and a phase diagram of a phase shift transformer (110) where a phase shift angle at a secondary side is negative to a phase shift angle at a primary side (Y/Z-2), when the primary side of the phase shift transformer (110) is formed in a Y winding of N1 turn, a secondary side is formed in a delta (Δ) winding of N2 winding, and a tertiary side is formed in a winding of N3.
As noted from the foregoing, an phase shift angle of the phase shift transformer (110) is determined by the number of unit power cells (120), where Ax, Bx, Cx of the unit power cells (120) respectively have a same phase shift angle. An output of secondary side of the phase shift transformer corresponds to the number of the diode rectifier (121) at the unit power cell (120), and a phase shift angle of the phase shift transformer (110) may be determined by the following Equation.
      α    sec    =      360          2      ⁢              N        sec            
where, unit of αsec is i [degree], and Nsec is the number of outputs at the secondary side of the phase shift transformer (110), or a total number of unit power cells (120). For example, Nsec is 9, and αsec is 20 in FIG. 1. Thus, an entire phase shift angle may be selected as 0°, 20°, −20° based on 0°.
When two unit power cells are used for each phase of a motor, Nsec is 6 and αsec is 30°. In this case, a phase shift angle may be selected as 0°, 30° based on 0°.
The conventional phased shift transformer thus described can output three pairs of secondary winding having a same phase shift angle, such that a problem of decreased THD in input phase current at the power side arises when the number of unit power cells for each phase of a motor is fewer than three.
This problem is caused by disability in selection of sufficiently small size of phase shift angle in the phase shift transformer (110), such that the conventional structure of phase shift transformer (110) disadvantageously generates a problem of satisfying the THD harmonics regulation at an input phase current in the system, only when more than three unit power cells for each phase of a motor are connected.